For many years it was the universal practice to construct roofs with a waterproof layer or membrane on the outer surface thereof. Such roofing is still used in many installations, but has many disadvantages. The waterproof membrane, which may be built-up sheet material and asphaltic or bitumin, or which may be a single sheet of waterproof material, is exposed to extreme temperature variations, as much as 200 degrees Fahrenheit, to ultraviolet radiation, and to physical abrasion, all of which have a detrimental effect on the life of the roofing.
It has been common practice for many years to provide insulation in roof construction, and when insulation is provided below the waterproof membrane, in the roofing system outlined above, it is necessary to provide a second waterproof membrane below the insulation to prevent moisture from within the building from condensing in the insulation and inhibiting or destroying the insulating qualities.
An alternative up-side-down roofing construction is known in which the insulation is applied over the waterproof membrane, see for example U.S. Pat. Nos. 3,411,256 and 3,763,614. In this alternative roof construction the waterproof membrane, which may be a built-up membrane or a single waterproof layer such as a thermoplastic or an elastomer, is applied directly to the surface of the roof. Blocks of foam plastic insulation are then applied over the waterproof membrane. STYROFOAM (Trademark of The Dow Chemical Company) brand foam polystyrene plastic insulation is a superior product for such use. It is a tough, closed-cell, rigid plastic foam having excellent moisture resistance and high compressive strength.
The foam polystyrene plastic insulation placed over the waterproofing membrane rather than under the membrane protects the membrane from the effects of thermocycling, temperature extremes, and physical abuse, thus reducing maintenance and prolonging the life of the entire roofing system. It has been found that the membrane so protected remains at stable temperatures below 100 degrees F. even in hot summer weather. In fact, under normal conditions, the temperature of the membrane will remain within 15-20 degrees F. of the building's inside temperature.
Typically, a polymeric fabric is installed over the foam to stabilize the system, and crushed stone or gravel ballast is applied to counteract the buoyancy of the insulation boards, to provide flammability resistance to the roof surface, and to shield the foam and fabric from ultraviolet radiation. As an alternative, paving blocks may be used in place of stone, particularly if traffic is to be expected on the roof. One such roofing system has been disclosed in co-pending application Ser. No. 639,751, filed Aug. 10, 1984 by David L. Roodvoets, and assigned to the same assignee as the present application, The Dow Chemical Company of Midland, Mich., U.S.A.
In some such inverted roofing installations utilizing lightweight concrete paving blocks over foam plastic insulation, there is a tendency for the wind to lift the paving blocks and insulation, and even to blow them from the roof deck. This is particularly true if each paving block and insulation slab is not thoroughly anchored to the roof deck. The situation can be particularly bad when the foam insulation and lightweight paving blocks are placed over unattached, single-ply membranes. The single-ply membranes can be pressurized from below, due to building pressures and/or wind pressure getting beneath the membrane. When such pressurization occurs, the membrane will tend to billow or to form a balloon, and dump the paving blocks and insulation off the surface and expose them to the wind.